Along with the recent development of high-density and highly integrated electrical and electronic components, there exists a demand for improvement of the reliability of these components.
In particular, not only a moisture-proof property at room temperature but also high reliability even in a high-temperature and wet-heat environment is required of electrical and electronic components used in vehicle engines, water heaters, etc.
For the purpose of protecting such electrical and electronic components from moisture, dust-containing atmosphere, vibration, impact, etc., electrically insulating sealing members have been used. Flexible resins that have low hardness, e.g., silicone-based resins and urethane-based resins, have typically been used as materials for such sealing members.
Silicone-based resins are excellent in heat resistance, flexibility, and low-temperature characteristics. However, silicone-based resins are not necessarily satisfactory in adhesion to materials of electrical components, and the moisture permeability is high. Thus, silicone-based resins are disadvantageous in that the influence of water cannot be fully avoided.
On the other hand, urethane-based resins are inherently excellent in flexibility, abrasion resistance, low-temperature curability, electrical characteristics, and the like, and are thus used for electrically insulating sealing members.
MDI (diphenylmethane diisocyanate) is typically used as an isocyanate component of urethane-based resins. Such urethane-based resins are often poor in heat resistance. Thus, unfortunately, when such urethane-based resins are used as a sealing member of an electrical component, particularly in a severe environment (e.g., in the vicinity of an engine), cracks are likely to appear on the surface of the sealing member over time, and the electrical component cannot be protected from moisture over a prolonged period of time.
As a heat-resistant polyurethane resin composition for electrical components, there have been proposed, for example, aliphatic and/or alicyclic polyurethane resins for sealing solar cells that are electrically connected on a panel (see Patent Document 1).
However, an analysis of the compatibility between the polyisocyanate component and the polyol component is not conducted in Patent Document 1. An isocyanurate ring-containing polyisocyanate has poor compatibility with a polyol component. Thus, the polyurethane resin composition proposed in Patent Document 1 has a drawback in that, when the polyurethane resin composition is used, the resulting cured molded article becomes sticky; and adhesion to an object becomes weak, which leads to a decrease in moisture resistance and insulation properties.
As a heat-resistant polyurethane resin composition in which a polyisocyanate component is compatible with a polyol component, Patent Document 2 proposes a polyurethane resin composition comprising a polybutadiene polyol (A) and a castor oil-based polyol (B) as hydroxyl-containing compounds, and a modified isocyanurate (c) of a polyisocyanate compound as an isocyanate-containing compound. Patent Document 3 proposes a polyurethane resin composition that is obtained by reacting a polyisocyanate (A) with a polybutadiene polyol (B), wherein the 1,2-vinyl structure content in the polybutadiene polyol (B) is more than 85 mol %.
However, in both Patent Documents 2 and 3 as well, an analysis of the polyisocyanate component for obtaining a polyurethane resin composition excellent in compatibility is not fully conducted. Due to the insufficient compatibility between the polyisocyanate component and the polyol component, it cannot be said that heat resistance, moisture resistance, and insulation properties are satisfactorily achieved in the use of such polyurethane resin compositions for electronic components. Therefore, to be suitably used for various electrical components, these polyurethane resin compositions remain to be improved.